1. Field of the Invention
The present invention relates to a novel photochromic compound and an optical function device using the same. To be more specific, it relates to a novel photochromic compound that is based on a diarylethene structure and capable of forming an amorphous thin film by itself, and an optical function device suitable for optical storage and switching.
2. Description of the Prior Art
As indicated in the following formula, photochromic compound means a molecule or molecule aggregation capable of transforming reversibly between two different forms, i.e., isomers A and B, in response to light irradiation. In other words, as shown in the following formula, isomer A changes, i.e., isomerizes, into isomer B by irradiation of light having a wavelength of xcex1, and isomer B retransforms into isomer A by irradiation of light having a wavelength of xcex2. 
Since isomers A and B show different optical properties such as absorptivity, refractive index, optical rotatory power and permittivity, the above-mentioned reversible optical response phenomenon makes it possible to apply the compound to a memory material. In a specific case of using the compound for a memory, information can be recorded through transformation of isomer A into isomer B by irradiation of light having a wavelength (xcex1). The recorded information can be read based on the difference of optical properties including absorbance and refractive index between the two isomers (i.e., isomers A and B). Moreover, the recorded information can be erased through retransformation of isomer B into isomer A by irradiation of light having another wavelength (xcex2).
To use the photochromic compound for optical recording including an optical memory, it is required for the photochromic compound; 1) to be an excellent in storage of information, that is, the isomers of the compound, A and B, have excellent thermal and chemical stabilities; 2) to have an excellent durability against repeating operations of writing and erasing, that is, the above-mentioned reversible reaction can be performed repeatedly: 3) to have a large difference in refractive index or absorbance between the isomers A and B sufficient to obtain a high C/N ratio reading signal; and 4) to form a thin film easily.
As the photochromic compound excellent in thermostability and durability against repeating its isomerization, diarylethene derivatives have been studied in recent years. The diarylethene derivatives undergo ring-opening and ring-closing reactions, as shown in the following formula, to be colored and bleached respectively. In addition, both of isomers, i.e., the open-ring and closed-ring forms, of the diarylethene derivatives are thermally stable. 
In known derivatives shown in the above formula, X and Y are respectively one of oxygen atom, sulfur atom and nitrogen atom of secondary or tertiary amine; R1 and R2 are each alkyl group; R3, R4, R5 and R6 are each one selected from a group consisting of alkyl group, alkoxyl group and hydrogen atom.
It is inevitable for the conventional photochromic compound based on diarylethene derivatives to be dispersed into a polymer matrix such as polymethylmethacrylate and polystyrene in order to form a thin film. Such a photochromic compound thin film needs to show large changes in absorbance and refractive index between the isomers of the compound enough to obtain a reading signal having a high C/N ratio. It is therefore important to increase molecular density of the photochromic compound contained in the film. However, no more than 30 parts by weight of the photochromic compound can uniformly disperse into 100 parts by weight of the polymer matrix because of the mutual-solubility between the photochromic compound and the polymer matrix, resulting in obtaining a thin film having poor properties. Therefore, it has been desired to develop a photochromic compound that is capable of forming a thin film by itself.
M. Irie, Chem. Lett. 899(1995) and Japanese Unexamined Patent Publication No. 8-119963 disclose a photochromic compound capable of forming a thin film by itself without using a polymer matrix. In the disclosure, the photochromic compound is deposited to a substrate to form a crystallized film of the photochromic compound. Such a formation of the thin film through vapor deposition, however, is disadvantageous in a mass-production. For the mass-production of the film, it has been desired to form the photochromic compound thin film by a coating method including spin coating.
Japanese Unexamined Patent Publication No. 9-241254 discloses a photochromic compound capable of forming a thin film without using a polymer matrix and, in addition, through a coating method, e.g., spin-coating. This reference proposes a photochromic compound in which at least one of the hetero-cyclic aryl groups constituting diarylethene derivative has an adamanthyl group. The photochromic compound can exhibit an amorphous solid state by itself without using a polymer matrix. In addition, it can form a thin film having the photochromic compound by itself through a coating method such as spin coating. However, this reference fails to disclose optical properties, e.g., refractive index, and thermostability of the compound. It is thus unknown whether the compound has favorable properties sufficient to be practically used for an optical recording device.
Jpn. J.Appl. Phys.38 (1999) L119 also discloses diarylethene photochromic compound that is capable of forming a thin film by itself. This compound, however, has a low glass transition temperature of about 70xc2x0 C. Thus, it is impossible to use the compound for an optical function device which requires a practical thermostability.
It is an object of the present invention to provide a photochromic compound capable of forming an amorphous thin film by itself without dispersing in a polymer resin matrix. The photochromic compound also can exhibit a large change of its optical properties such as absorbance and refractive index between before and after light irradiation. Furthermore, it is excellent in thermo- and chemical-stabilities, since it has a glass transition temperature of greater than 100xc2x0 C. It is another object of the present invention to provide an optical function device using the photochromic compound.
The inventors have found the followings. A volumed substituent bound to a hetero-cyclic aryl group constituting a diarylethene skeleton of a photochromic compound makes it possible to form an amorphous thin film made of the photochromic compound by itself. In addition, using a substituent having at least three aryls as the volumed substituent makes it possible to give an excellent thermostability, i.e., a glass transition temperature of greater than 100xc2x0 C., to the photochromic compound, to complete the present invention.
According to the preset invention, provided is a diarylethene photochromic compound shown in the following formulas (1) and (2), in which R5 and R6 are each organic group having at least three aryls. 
In the above formulas, ring B is a hetero-cyclic aryl group selected from a group consisting of furan, thiophene and pyrrole; ring A is at least one selected from a group consisting of alicyclic group, aromatic ring, maleic anhydride and maleimide group; ring C is a benzene ring or its derivatives; R1 and R2 are each one of hydrogen atom and an alkyl group having 1 to 6 carbon atoms, and they may be same or different from each other; and R3 and R4 are each one selected from a group consisting of hydrogen atom, an alkyl group having 1 to 6 carbon atoms, cyano group, aryl group, halogen atom, nitro group, amino group and alkoxyl group, and they may be same or different from each other.
According to the preset invention, also provided is an optical function device suitable for an optical memory using the above-mentioned photochromic compound according to the present invention. The optical function device comprises a substrate; and an amorphous thin film layer made of the above-mentioned photochromic compound provided on the substrate.
The preset invention further provides an optical function device suitable for an optical switching using the above-mentioned photochromic compound according to the present invention. The optical function device comprises a translucent substrate, a transparent plastic layer provided on the substrate; a first and a second optical waveguides formed inside the transparent plastic layer, each of which has a first and a second branch sections for branching light passing through the first and second optical waveguides. The photochromic compound thin film according to the present invention is used as a clad layer on the first optical waveguide between the first and the second branch sections thereof.
The term xe2x80x9cArylxe2x80x9d in this specification means a cyclic group having aromaticity that has a xcfx80 electron in accordance with the Huckel""s rule. The cyclic compounds include benzene ring, condensed ring, hetero ring and derivatives of these.
These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description.